When deleting a linkNode, the method deleteObject of ExtendedStore, update the
target object of the link by removing the to-be-deleted link from the list of links.
Extended store retrieve the object from the cache, but doesn't check if the
object is in transaction cache or global cache. If the target object has already
been modified in this transaction it is in the transaction cache and all goes
well, otherwise, the object is taken from the global cache and UPDATED. But when
in case of rollback, the global cache is not restored.

Conclusion: after a rolled back delete of a link, slide cache is no more
synchronized with the store.

Test: 
* create a node A
* create a link B to the node A
* [enumerate links of A] (result must be {'B'})
* begin transaction
* delete B
* [enumerate links of A] (result must be {})
* Rollback transaction
* [enumerate links of A] (result should be {'B'} but is {})